Impact of Organic Acids/Chelating Agents on the Rheological Properties of an Amidoamine-Oxide Surfactant

Abstract

Summary Surfactant-based acids have been used for diversion because they are less damaging to the formation. Amphoteric viscoelastic surfactant (VES) is the main type of surfactant being used today. Low viscosity is observed in the live-acid systems, whereas significantly increased viscosity is found when HCl reacts with carbonate and generates divalent salts. The surfactant/acid system can be broken after acid treatments by mixing with reservoir hydrocarbons, or by using an external or internal breaker. Amidoamine oxide, an amphoteric surfactant, was examined in this work. The prepared surfactant-based live-acid system contained 20 wt% HCl, 4 wt% surfactant, and 1 wt% corrosion inhibitor. Different organic acids/chelating agents were added to live and spent acids. Calcium carbonate particles were used to neutralize live acids. The objective was to examine how these organic acids/chelating agents affected the rheological properties of spent-acid systems. Measurements were made at temperatures from 75 to 200°F at a shear rate of 10 sec–1 and a pressure of 300 psi. Several simple organic acids (formic, acetic, propionic, and butyric) and chelating agents [glycolic acid, lactic acid, gluconic acid, citric acid, tetrasodium ethylenediamine tetraacetic acid (EDTA), tetrasodium L-glutamic acid, N, N-diacetic acid (GLDA), and disodium L-glutamic acid, N, N-diacetic acid (HEIDA)] that are used in the field were examined. Experimental results indicated that the addition of simple organic acids or chelating agents reduced the viscosity of spent acids significantly. This reduction in viscosity increased with the number of carbon atoms in the organic acid. The addition of organic acids reduced the temperature range in which the surfactant can be used. Chelating agents (α-hydroxy carboxylic acids and amino carboxylic acids) also tended to break the surfactant gel if enough chelant was used. On the basis of the results obtained, organic acids can be used to break surfactant gel. Transmission electron microscopy (TEM) tests were first conducted to examine how organic acids/chelating agents interfered with the formation of rod-like micelles in the surfactant-based acid. The results showed that the addition of organic acids to the spent acid generated a fewer elongated micelles and resulted in less apparent viscosity. In addition, if chelating agents or simple organic acids are used, then the concentration of the surfactant should be increased to compensate for the loss of viscosity induced by the addition of the organic acids.